Modern designs of aircraft propeller blades use composite materials to produce lightweight blades capable of supporting the operating loads. These operating loads include a centrifugal force component which acts in a direction parallel to the longitudinal axis of the propeller blade. Historically, retention systems of non-preloaded propeller blades rely on the centrifugal load generated by the weight of the blade structure during operation to stiffen the blade retention for load carrying capability, especially the steady and cyclic bending loads. As propeller blades have gotten lighter with the use of composite materials, the retention systems of non-preloaded propeller blades have been altered to provide the necessary load for stiffening of the blade retention. This alteration has resulted in large and heavy parts for retention mechanisms.
Modern propeller blade retention systems can address the problems associated with size and result in weight reduction. Many of these systems will use an assembly of bearings for retention of the propeller blade inside a hub assembly. See for example U.S. Pat. No. 4,921,403 which includes bearing arrangements utilizing roller bearings. Such a bearing arrangement retains the propeller blade from inside the hub and reduces the amount of vibration. However, this design does not provide means for preloading which is required by retention systems that include modern composite blades. There is prior art that addresses the need for preloading. U.S. Pat. No. 4,850,801 discloses a bearing arrangement with means for preloading. However, such an arrangement does not provide ease of replacement and maintenance of parts, since the blade is directly interfaced with the inside of the hub and the retention assembly. Blade replacement would require the removal of bearing parts which further complicates the maintenance process. Furthermore, the cost and complexity of tools necessary to perform routine maintenance increases. Interfacing the propeller blade with the retention parts places limitations on the types of blades that may be used with the system as well as requiring a more complex blade design at the root of the blade.
To address the need for flexibility of interfacing a plurality of propeller blades, some modern applications utilize a receiving means. In such an application the blade is interfaced only with the receiver. The receiver is then interfaced with a retention system. Thus the blade is simplified since it need not be designed for interfacing with the retention system. An example of utilization of a receiving means is shown in U.S. Pat. No. 4,863,352. Unlike the present design this patent utilizes roller bearings in retention and rotation of the propeller blade. Also, the patent, unlike the present invention, accomplishes preload of the system by using an annular fastener threaded onto the hub. While this type of retention mechanism addresses the problems of size and weight of retention mechanisms, it has the inherent limitation of introducing stress concentrations in the threads on the hub, which operates under high cyclic fatigue loading, which can result in hub failure. Similarly, U.S. Pat. No. 4,850,801, discussed above, utilizes a threaded fastener which interfaces with the threaded portion of the hub, thus introducing similar high stress on the hub. The present invention does not include threaded parts on any of the three major parts of the propeller assembly, the hub, the receiver, or the propeller blade.
There exists a need, therefore, for a propeller blade retention system which allows for a coexisting functionally separable blade and retention arrangement and which allows for retention mechanism preloading, yielding a simultaneous reduction in costs, parts, and maintenance complexity.